docs: import Proton Mail, Outlook and iCloud Mail sections back from v1.2.0

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Refs: #705

README.md

@@ -26,7 +26,10 @@
   - [Nix](#nix)
   - [Sources](#sources)
 - [Configuration](#configuration)
+  - [Proton Mail](#proton-mail)
   - [Gmail](#gmail)
+  - [Outlook](#outlook)
+  - [iCloud Mail](#icloud-mail)
 - [Usage](#usage)
   - [Shared API](#shared-api)
   - [Protocol-specific APIs](#protocol-specific-apis)
@@ -186,6 +189,32 @@ Override the path with `-c <PATH>`; multiple paths can be passed at once, separa
 
 Accounts can be (re)configured later with `himalaya account configure <name>`. The wizard skips discovery in this mode: it reuses the existing values as prompt defaults.
 
+### Proton Mail
+
+Proton does not expose IMAP/SMTP directly: run [Proton Bridge](https://proton.me/mail/bridge), which synchronizes mail locally and serves it on a local IMAP/SMTP endpoint. The password is the one generated by the Bridge, not your Proton account password.
+
+```toml
+[accounts.proton]
+
+imap.server = "imap://127.0.0.1:1143"
+imap.sasl.plain.username = "example@proton.me"
+imap.sasl.plain.password.command = "pass show proton-bridge"
+
+smtp.server = "smtp://127.0.0.1:1025"
+smtp.sasl.plain.username = "example@proton.me"
+smtp.sasl.plain.password.command = "pass show proton-bridge"
+```
+
+To keep TLS on the local link, export the certificate generated by the Bridge and enable STARTTLS:
+
+```toml
+imap.starttls = true
+imap.tls.cert = "/path/to/exported/cert.pem"
+
+smtp.starttls = true
+smtp.tls.cert = "/path/to/exported/cert.pem"
+```
+
 ### Gmail
 
 Gmail rejects the account password over SASL PLAIN: generate an [app password](https://myaccount.google.com/apppasswords) (requires 2-step verification) and feed it through `password.command` or `password.raw`.
@@ -211,6 +240,42 @@ mailbox.alias.archive = "[Gmail]/All Mail"
 
 Every Gmail label shows up as a top-level IMAP mailbox, and the special mailboxes live under the `[Gmail]/` prefix — quote them in the shell (`-m "[Gmail]/Drafts"`) or reach them through an alias. `[Gmail]/All Mail` is the archive containing every message: aliasing it makes "search everything" one flag away (`himalaya envelope search -m archive ...`).
 
+### Outlook
+
+Microsoft has retired basic authentication: use OAuth 2.0 via `oauthbearer` or `xoauth2`, with the access token supplied by an external helper such as [ortie](https://github.com/pimalaya/ortie).
+
+```toml
+[accounts.outlook]
+
+imap.server = "imaps://outlook.office365.com:993"
+imap.sasl.xoauth2.username = "example@outlook.com"
+imap.sasl.xoauth2.token.command = ["ortie", "token", "read", "outlook"]
+
+smtp.server = "smtp://smtp-mail.outlook.com:587"
+smtp.starttls = true
+smtp.sasl.xoauth2.username = "example@outlook.com"
+smtp.sasl.xoauth2.token.command = ["ortie", "token", "read", "outlook"]
+```
+
+### iCloud Mail
+
+From the [iCloud Mail](https://support.apple.com/en-us/HT202304) support page: the IMAP login is the name of your address (`johnappleseed`, not `johnappleseed@icloud.com`) while the SMTP login is the full address, and a dedicated [app-specific password](https://support.apple.com/en-us/HT204397) is required.
+
+```toml
+[accounts.icloud]
+
+imap.server = "imaps://imap.mail.me.com:993"
+imap.sasl.plain.username = "johnappleseed"
+imap.sasl.plain.password.command = "pass show icloud"
+
+smtp.server = "smtp://smtp.mail.me.com:587"
+smtp.starttls = true
+smtp.sasl.plain.username = "johnappleseed@icloud.com"
+smtp.sasl.plain.password.command = "pass show icloud"
+
+mailbox.alias.sent = "Sent Messages"
+```
+
 ## Usage
 
 ### Shared API